Strange behaviour of gdb - c++

Question

I am debugging following sample code in gdb
#include <iostream>
#include <string.h>
using namespace std;
void c (char** q)
{
q = new char*[2];
if (q == NULL)
cout<<"NO OK";
else
cout<<"OK";
}
int main ()
{
char** d = NULL;
c(d);
return 1;
}
Now this program gives output as "OK"
but while debugging this code in gdb i found following behavior
gdb -version
GNU gdb (GDB) 7.5.1
Breakpoint 1, c (q=0x0) at issue.cpp:8
warning: Source file is more recent than executable.
8 q = new char*[2];
(gdb) p q
$1 = (char **) 0x0
(gdb) n
9 if (q == NULL)
(gdb) p q
$2 = (char **) 0x0
(gdb) p q==0
$3 = true
(gdb) n
12 cout<<"OK";
(gdb) p q
$4 = (char **) 0x0
(gdb) p q==0
$5 = true
(gdb)
gdb is showing q value a null but code is executing differently.

Answer 1

Well: warning: Source file is more recent than executable. - That may be your answer. Try recompiling your code.

Answer 2

There is definitely something going on that messes up with the gdb logic. I am encountering your described behavior in gcc 4.8.3 running in cygwin (32 bits).
I am getting this output in my gdb session for your code:
Breakpoint 1, main () at test.cpp:21
21 char** d = NULL;
(gdb) s
22 c(d);
(gdb) s
c (q=0x0) at test.cpp:9
9 q = new char*[2];
(gdb) s
13 if (q == NULL)
(gdb) p q
$1 = (char **) 0x0
(gdb) p &q
$2 = (char ***) 0x22abd0
(gdb) x 0x22abd0
0x22abd0: 0x00000000
Now if I simply change your function like this:
void c (char** q)
{
cout << q << endl;
q = new char*[2];
if (q == NULL)
cout<<"NO OK";
else
cout<<"OK";
}
gdb seems now capable of getting that q parameter's value:
Breakpoint 1, main () at test.cpp:21
21 char** d = NULL;
(gdb) s
22 c(d);
(gdb) s
c (q=0x0) at test.cpp:8
8 cout << q << endl;
(gdb) s
0
9 q = new char*[2];
(gdb) s
13 if (q == NULL)
(gdb) p q
$1 = (char **) 0x2004a0a8
(gdb) p &q
$2 = (char ***) 0x22abd0
(gdb) x 0x22abd0
0x22abd0: 0x2004a0a8
Let's change the code like this:
void c (char** q)
{
q = new char*[2];
cout << q << endl;
if (q == NULL)
cout<<"NO OK";
else
cout<<"OK";
}
The generated code for this function looks something like this: (compiled -g -ggdb -O0)
.cfi_startproc
pushl %ebp
.cfi_def_cfa_offset 8
.cfi_offset 5, -8
movl %esp, %ebp
.cfi_def_cfa_register 5
subl $40, %esp
movl $8, (%esp)
call __Znaj
movl %eax, -12(%ebp)
movl -12(%ebp), %eax
movl %eax, 4(%esp)
movl $__ZSt4cout, (%esp)
call __ZNSolsEPKv
movl $__ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_, 4(%esp)
movl %eax, (%esp)
call __ZNSolsEPFRSoS_E
cmpl $0, -12(%ebp)
jne L2
movl $LC0, 4(%esp)
movl $__ZSt4cout, (%esp)
call __ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc
jmp L1
Now it seems from this code that the compiler uses for if (q == NULL) the value at $ebp - 12. Let's see how this goes during the gdb session:
Breakpoint 1, main () at test.cpp:19
19 char** d = NULL;
(gdb) s
20 c(d);
(gdb) s
c (q=0x0) at test.cpp:8
8 q = new char*[2];
(gdb) s
9 cout << q << endl;
(gdb) s
0x2003a078
11 if (q == NULL)
(gdb) p q
$1 = (char **) 0x0
(gdb) p &q
$2 = (char ***) 0x22abd0
(gdb) p $ebp
$3 = (void *) 0x22abc8
(gdb) p $ebp - 12
$4 = (void *) 0x22abbc
(gdb) x 0x22abbc
0x22abbc: 0x2003a078
Looks like gdb looks for q at a different address than the one it actually is at. You probably just got yourself into a delicate gcc - gdb interaction issue.

Answer 3

Normally debugging is done on the non-optimized executable (and measuring performance - on optimized executable). This is because doing things fast and doing them exactly the way want them to do is mutually exclusive.
Your c function can be rewritten to
void c (char** q)
{
cout<<"OK";
}
This is because you assign the newed array on the function argument, that is, the local variable. This means the code has no side effects to the rest of the world. Also, new never returns NULL, and so q == 0 is always false.
EDIT: Ok, it's actually more complicated than a simple debug/release difference. Since the actual problem lies elsewhere, to make this answer more useful, this is how it should behave (code compiled with g++ 4.9.0, and gdb version is 7.7)
Non-optimized g++ -g
Breakpoint 1, c (q=0x0) at a.cpp:8
8 q = new char*[2];
(gdb) p q
$1 = (char **) 0x0
(gdb) n
9 if (q == NULL)
(gdb) p q
$2 = (char **) 0x611290
(gdb) p q==0
$3 = false
(gdb) n
12 cout<<"OK";
(gdb) p q
$4 = (char **) 0x611290
(gdb) p q==0
$5 = false
Optimized g++ -g -O2
Breakpoint 1, c (q=0x0) at a.cpp:8
8 q = new char*[2];
(gdb) p q
$1 = (char **) 0x0
(gdb) n
12 cout<<"OK";
(gdb) p q
$2 = <optimized out>
(gdb)